The present invention relates to a rubber composition for a tire tread, and particularly relates to a rubber composition for a tire tread improving a tire in a performance on snow and ice road without decreasing in abrasion resistance.
A large number of patents disclosed that mixing short fibers (synthetic fibers, natural fibers, glass fibers, carbon fibers and the like) with a rubber improves the performance on snow and ice road and the abrasion resistance. Among these short fibers, inorganic fibers, for example, glass fibers can improve the performance. This is caused by that glass fibers are harder than ice and scratch ice. On the contrary, organic fibers (pulp, polyethylene, polyester, nylon and the like) are softer than ice and not expected to scratch ice.
However, inorganic fibers are hard materials, and tend to lead the rubber composition to hard in proportion to the amount thereof.
Generally, the hardness is adjusted by using liquid components such as petroleum softeners (aromatic oil, naphthene oil, paraffin oil, and the like) and low temperature plasticizers (dioctyl phthalate (DOP), dibutyl phthalate (DBP) and the like).
An increased amount of the liquid components are expected to decrease dispersibility of reinforcing agents (carbon black, silica and the like). The decreased dispersibility of reinforcing agents does not provide rubber property as planed, and provide decreased performance on ice road and decreased abrasion resistance.
For example, it is known that a spike tire is increased in performance on snow and ice road and abrasion resistance by using a rubber composition for a spike pin obtained by mixing polyamide short fibers or aramide short fibers with a rubber (e.g., Japanese unexamined patent publication No. 109618/1997). However, the tire of the rubber composition comprising polyamide short fibers or aramide short fibers is a spike tire having a problem in damage of a road surface. Further, organic fibers require a step for cutting long fibers to short fibers. Furthermore, organic fibers have a large number of problems in work environment or the like because of a low specific gravity and tendency to easily splash, and a problem of insufficient dispersion because of difficulty in dispersing in a mixing step.
Those are known that a tire showing a high friction force on snow and ice road, a low damage on a road surface, and a high tire performance can be produced from a rubber composition obtained by mixing cured powders of a hardening resin with a rubber (Japanese unexamined patent publication No. 278941/1997), that a studless tire having an excellent grip performance on ice road can be produced from a tread rubber composition obtained by mixing a silicone polymer with a rubber (Japanese unexamined patent publication No. 241427/1997), that a pneumatic tire having an excellent performance on snow and ice road and wet skid characteristics can be obtained by forming a foam rubber layer of a rubber composition comprising silica, carbon black or the like (Japanese unexamined patent publication No. 258469/1995).
However, the tire of a rubber composition obtained by mixing cured powders of a hardening resin has an insufficient performance on snow and ice road and a decreased abrasion resistance because the powders fall easily from the rubber composition. The tire of the tread rubber composition comprising a silicone polymer has a lower performance on snow and ice road than that comprising glass fibers.
The tire with a foam rubber layer of a rubber composition comprising silica, carbon black or the like has a problem of increasing in rubber hardness and decreasing in performance on snow and ice road with the passage of time when the ultrafine powders of reinforcing agents (carbon black, or silica) are used for improving the decreased abrasion resistance.